mcp-atlassian crashes with Pydantic 2.12.0: 'cannot specify both default and default_factory'

Question

After Pydantic 2.12.0 was released, running uvx mcp-atlassian on Linux with Python 3.12 fails immediately with TypeError: cannot specify both default and default_factory. The crash affects the main branch and the released PyPI package, and was also seen from gemini-cli. Why does this happen and what is the workaround?

Originally asked on GitHub: https://github.com/sooperset/mcp-atlassian/issues/721

AgentArmor team · Accepted Answer

The thread explains the root cause: mcp-atlassian's pinned `fastmcp` dependency defines a setting with both `default` and `default_factory`, which Pydantic 2.12.0 rejects. The fastmcp problem is fixed upstream, so mcp-atlassian needs a newer `fastmcp` constraint; the thread does not name a fixed mcp-atlassian release. Until then, pin Pydantic below 2.12.

Workaround that was confirmed: replace `"pydantic>=2.10.6"` with `"pydantic==2.11.9"` in `pyproject.toml` and reinstall. If you use `uvx`, set the MCP server args in your client config:

```json
{
  "mcpServers": {
    "mcp-atlassian": {
      "command": "uvx",
      "args": ["--with", "pydantic==2.11.9", "mcp-atlassian"],
      "env": {}
    }
  }
}
```

This exact uvx config also succeeded for another commenter. A contributor suggested using `"pydantic<2.12.0"` instead of pinning an exact version, but that form was not confirmed in the thread. Avoid `pydantic==2.7.2`; one user got `ImportError: cannot import name 'elements_from_string' from 'md2conf.converter'` with it.

Other workarounds had mixed results: upgrading uv from 0.8.24 to 0.9.0 helped one user but not another, and switching the client command from `uvx` to `mcp-atlassian` was reported to work by one user. Also note that Pydantic versions below 2.12 do not support Python 3.14, so Python 3.14 users may need to wait for a fixed release.

AgentArmor team · Answer

**What @manishyadav7 tried on GitHub** (Worked)

Pinned `"pydantic==2.11.9"` in `pyproject.toml` (replacing `"pydantic>=2.10.6"`) and reinstalled; the server ran successfully.

Source: https://github.com/sooperset/mcp-atlassian/issues/721#issuecomment-3381064304

AgentArmor team · Answer

**What @staywen56 tried on GitHub** (Worked)

Used the MCP config `uvx --with pydantic==2.11.9 mcp-atlassian`; worked for this commenter, while `pydantic==2.7.2` produced an md2conf ImportError.

Source: https://github.com/sooperset/mcp-atlassian/issues/721#issuecomment-3403845815

AgentArmor team · Answer

**What @aaronchiang tried on GitHub** (Partly worked)

Upgraded uv from 0.8.24 to 0.9.0 and reverted to the default `uvx mcp-atlassian` config. Worked for this user, but comment 4 reported the same upgrade did not help.

Source: https://github.com/sooperset/mcp-atlassian/issues/721#issuecomment-3380429380

AgentArmor team · Answer

**What @osechet tried on GitHub** (Outcome unclear)

Proposed by a contributor: use `uvx --with pydantic<2.12.0 mcp-atlassian` instead of pinning an exact version. No later comment confirmed it.

Source: https://github.com/sooperset/mcp-atlassian/issues/721#issuecomment-3405125937

AgentArmor team · Answer

**What a GitHub user tried on GitHub** (Didn't work)

Reproduced the bug with `uvx mcp-atlassian` on Linux/Python 3.12; it failed with `TypeError: cannot specify both default and default_factory`.

Source: https://github.com/sooperset/mcp-atlassian/issues/721

mcp-atlassian crashes with Pydantic 2.12.0: 'cannot specify both default and default_factory'

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mcp-atlassian crashes with Pydantic 2.12.0: 'cannot specify both default and default_factory'

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